text:
Should I open a door or window when there is smoke in the cockpit?
Let's say when flying, I encounter smoke and the smell of something burnt in the cockpit. Should I open the door or a window to clear the smoke?
My thoughts: the smoke may be toxic; opening the door brings in fresh air and avoid incapacitation. But, if something is burning, opening the door also brings in plenty of oxygen, and the fire will spread very rapidly.
I'm asking about GA planes.

You should focus the question on GA because the airline answer is uniformly "don oxygen mask, and smoke goggles, land at nearest suitable airport" and then follow the QRH to start pulling main bus breakers to isolate the electrical fault.

What does the P.O.H. for the aircraft you're flying have to say about this?

During a fire on board SAA flight 295, a 747 Combi, the procedures called for them to ascend to a low altitude and open two of the cabin doors to evacuate the smoke.

If possible, you should do whatever the operating handbook for your aircraft says but don't hesitate to do something different if you think it's necessary (more on that below).
I checked a couple of POHs that I have to hand and they don't mention the doors specifically, but there is a slight difference in what they say about cabin ventilation.
The C172S checklist emphasizes avoiding drafts, including shutting air vents and ventilating only after fighting the fire:

ELECTRICAL FIRE IN FLIGHT

Master Switch -- OFF.
Vents, Cabin Air, Heat -- CLOSED.
Fire Extinguisher -- ACTIVATE.
Avionics Master Switch -- OFF.
All Other Switches (except ignition switch) -- OFF.

WARNING
AFTER DISCHARGING FIRE EXTINGUISHER AND ASCERTAINING THAT FIRE
  HAS BEEN EXTINGUISHED, VENTILATE THE CABIN.

Vents/Cabin Air/Heat -- OPEN when it is ascertained that fire is completely extinguished.
  [...]

The PA28 checklist on the other hand says that the vents should be open:

FIRE IN FLIGHT
Electrical fire (smoke in cabin):
  Master switch....OFF
  Vents....open
  Cabin heat....OFF
  Land as soon as practicable

But practically speaking, fire in the cabin is a really extreme situation and it's hard to say that you should always follow the checklist (if you even physically can). For example, if the cabin fills with smoke and you can't see anything at all you might have to try to clear the smoke somehow just to keep control of the aircraft, even at the risk of feeding the fire. The Piper POH acknowledges that the pilot is the best person to decide what to do:

The procedure given is general and pilot judgement should be the
  determining factor for action in such an emergency

For a real-life account of a cabin fire in a C172RG, see this AOPA Real Pilot Story. They had to open the door to be able to see and land, which reinforces the point about doing whatever you have to do in an extreme scenario.

Clearly the difference between the checklists is that the first assumes availability of fire extinguisher (the word “activate” suggests there might even be a built-in extinguisher for the electronics bay) and the later does not. Presumably the extinguisher is a halon one designed for use in closed spaces (some halon agents are effective at 7-9% concentration, leaving enough oxygen for humans to breathe) and having vents open while using the extinguisher would just blow out the extinguishing agent. While when not extinguishing, the smoke has to be vented so the crew can still see and breathe.

Burning smells in an aircraft always need to be taken seriously, not least because it could be a sign that you're breathing in carbon monoxide, which could effectively incapacitate you without you even realising.
In an unpressurised aircraft you wouldn't exactly be bringing in more oxygen, so to speak, as the cabin altitude is already the same as outside. But yes you would be adding wind to a flame which could spread it instead of extinguishing it, but IMO this risk is worth taking compared to the alternative.
I'm not sure if there's a formal recommended procedure for all aircraft, but it would be prudent to initially look around and see if there is a visible flame. If not, you must close all air conditioning vents and open the windows. If you see smoke you should land immediately in a field. If not, divert to the nearest airfield, ensuring the fresh wind is blowing in your face.

You'd be bringing in fresh oxygen if some inside the aircraft is being used up by a fire, though...


text:
Are helicopters aerodynamically stable?
On a fixed wing aircraft, if the aircraft's attitude is disrupted by a small angle and the controls are let go, it will slowly right itself to straight and level, due to built-in aerodynamic stability (on most aircraft).
If a helicopter is banked or pitched a small angle, will it right itself with the horizon if the controls are let go?

It's complicated ;)
There are two types of stability; dynamic and static.
If an aircraft is disturbed by, say, a gust of wind, it will deviate from its attitude but then will immediately and without control inputs return to its original attitude.  This is positive static stability.  If it remains in the disturbed attitude unless corrected, it has neutral static stability.  If it continues to deviate, it has negative static stability.
If the aircraft returns to its original attitude, overshoots a bit, then corrects back, overshoots a little bit less and so on, eventually returning to its original attitude, then it has positive dynamic stability.  If these oscillations continue and do not "damp" down until the original attitude is regained, then it has neutral dynamic stability.  If the oscillations grow increasingly large, then it has negative dynamic stability.
For helicopters, I will ignore those with autopilots, hydraulic controls and stability augmentation systems.
Imagine hovering.  A gust of wind hits the helicopter.  The disk will "flap" away from the gust and the helicopter will move with the gust.  When the gust stops, the disk now experiences airflow from the opposite direction, since it is now moving sideways.  The disk will flap back the other way, returning to neutral but the fuselage acts as a pendulum hanging under the disk and its momentum will cause it to swing further out, tilting the disk more and causing the helicopter to move faster in the opposite direction to the gust than is required to return to the original attitude.  Eventually, the airflow now comes from the direction the helicopter is moving in so the disk flaps away, the fuselage swings too far and......  Unless corrected, it will continue to swing backwards and forwards, rolling, yawing and pitching.  It is statically stable (because it tends to return to the original attitude) but dynamically unstable since it continues to swing backwards and forwards from the original attitude.
In forward flight (and unless you want to know, I'll skip the details which include what direction the gust comes from, the tendency of the fuselage to weathercock and "blowback" since the details all about which way the disk flaps and which way the fuselage weathercocks and are a lot of words!), a helicopter is statically stable and dynamically stable in yaw and is statically stable and dynamically unstable in pitch and roll.
You can take your feet of the pedals and the aircraft will be stable in yaw.  You can take your hand off the collective (although you should never do this - if the engine quits, you need that lever down fast) and it will stay where it is, ignoring perhaps vibration making it eventually move up or down.
You cannot take your hand off the cyclic since this is the control which determines pitch and roll and corrects for dynamic instability on a constant basis.
On a light helicopter, you would be out of control within a second or two.  On a heavy, maybe three or four seconds.
In summary, a helicopter is statically stable and dynamically unstable in all three axes in hover and forward flight except yaw in forward flight where it is dynamically stable.

Russia's Kamov claims that their helicopters are the only stable ones. I have flown one only once (and briefly, from the right side - and yes, you can take your hand off the stick for quite a while without anything bad happening), but I have too little helicopter knowledge to explain it. Maybe you can add a line on them?

@PeterKämpf Wow, I'd love to fly a Kamov.  But sadly, I have no idea how they do it.  Do you mean that the aircraft is dynamically stable or that taking your hands-off the cyclic doesn't do anything?  If it's the latter, I suspect maybe hydraulic augmentation.  I'll see if I can find something.  I'm wondering what the contra-rotating disks do for it.

I think the two disks cancel each other's side forces so no pendulum swinging occurs. I did not let go of the collective, just the stick and waited if the craft starts to roll but it stayed on course and straight. It was a Kamov 26, the one with radial engines on both sides.

I couldn't find anything authoritative. I think you're right.  A gust will cause an increase in angle of attack on one disk and a decrease on the other stabilising the disks and also resisting the yaw since the torque increases on the disk with a higher angle of attack and reduces on the other.

Sorry for asking again - doesn't a gust from the side cause a pitch motion of the rotor disk and vice versa? So any gust would start a cross-coupled reaction in both pitch and roll (since the inclination in pitch causes a longitudinal movement which in turn causes a rolling moment, and so on). Or are motions in each axis separated?

@PeterKämpf The pitch reaction depends on the wind direction.  The lift on the blade advancing into the gust will increase and flap up 90 degrees later.  A gust from the side causes a roll away from the gust and a pitch up.  I'm not sure what you mean by "are the motions separated".  Without control input a change in any axis will cause a change in the other two, either through aerodynamic effects or power changes.

Kamovs have stability augmenting autopilots. They have it for Pitch, Roll, Yaw and Altitude (collective), they provide 20% of input value and the other 80% is from the pilot. This is enough to correct small deviations that would otherwise compound over time. They are divided in channels and you can turn them on/off. Even if trimmed correctly it will not be able to keep straight for very long when all autopilots are off. Otherwise you can completely remove your hand and it will stay stable in what ever attitude it was before. If you want hover, there is a auto-hover additional autopilot.

@PeterKämpf do you have any further reading for the kamov stability?

@ManuH: No. I heard about it very long ago and cannot remember where. I would have to google this myself.

Aerodynamic stability is defined as:

statically stable when upon a disturbance from an equilibrium, an aerodynamic torque appears that will make the aircraft want to return to the equilibrium.
dynamically stable if there is a motion returning to the equilibrium position, with enough damping to settle into the new equilibrium position.

So static stability considers forces, dynamic stability considers motion responses over time due to forces. Static stability is a necessary condition for dynamic stability.
Helicopters are aerodynamically unstable in pitch and roll in the hover, as illustrated in this answer. In yaw, it will weathervane in the hover.
Stability in forward flight:

Pitch: statically unstable, unless a horizontal stabiliser is mounted which provides static and dynamic stability increasing with forward velocity.
Roll: statically unstable. However there is roll/sideslip stabilisation from the rotor coning angle, analogous to dihedral for fixed wing aircraft.
Yaw: statically and dynamically stable. Clear to envision if a vertical tail is mounted - if not, the tail rotor provides exactly the same stabilising tendencies as a vertical tail.

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text:
How to relay an emergency to ATC while on the ground?
So we have somebody that contacted ATC via a Facebook post. Suppose I saw that Facebook post and believed it to be a genuine emergency. How can I contact ATC?

You could phone the tower.

@RonBeyer Did you read the post, because he clearly explains that his first attempt was a phone call. It's entirely possible to have data connectivity without the ability to make and receive reliable calls. Better to drop out a quick post whicb can auto retry rather than to distract yourself with a lost cause.

@Dan I did, but maybe I missed that part, Facebook isn't known for its long readablity... There are a lot of places that you can also text 911 (although it isn't implemented country-wide yet).

@GregHewgill, how, exactly, do I look up the phone number to the tower?

@kevin: How can I find telephone numbers for FAA ATC facilities?

If you're in France you can diall 191 to contact the Rescue Coordination Center. http://www.aerovfr.com/2017/01/faites-le-191-pour-les-urgences-aeronautiques/

Call the police if in doubt who to call they have contact with Air Traffic Control.
Also calling the wrong facility can mean that it can take quite a while for the information to make it to correct facility.
Finding direct line phone numbers to ATC facilities can be intentionally hard. Prank call anyone! They are usually somewhere in the local Aeronautical Publication Books but where you can get a copy of them and then finding the number inside the book is difficult. Just call the police!

In a recent conversation with an FAA DPE, he was telling a bunch of us about a plane crash he witnessed as it happened (he was at the field where the airplane was attempting to make an approach in dense fog, which eventually resulted in a crash).
The DPE's first instinct was not to call 911, as he saw others in the vicinity doing that. He instead called the tower of the nearest CLASS C Airport (KDAB) with whom the aircraft would be in control of under an IFR Flight Plan.
He had KDAB tower's number on his cell (DPEs usually carry numbers of all nearby towers and important Aviation Authority Offices such as FSDO, NTSB etc.). If he didn't, it's not hard to get the number these days. Internet, ForeFlight, AFD - many resources.
Why did he call the tower? Several reasons:

He reported the crash to them (the pilots, according to him, didn't look like they would have survived the spin that resulted in a crash)
He told the tower not to expect a phone call to close the IFR Flight Plan (regulations would require a Search and Rescue operation within an hour after the ETA on the flight plan unless closed)
The tower would have more information about the Pilot and any contact number at the destination (from the Flight Plan). This would help the authorities on the ground.

Point is: who to call, when to call, how to call and what to report would depend on the nature of the emergency.
E.g.: If you're in a remote place with no one else around and you witness an emergency, your first call should be to get help.
Hope that answers the question.

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text:
How do the cockpit controls of a tiltrotor aircraft work?
I'm having trouble trying to imagine the primary flight controls in the cockpit of a tiltrotor aircraft - how would one design it?
In an airplane, turning the wheel to the left would always deflect the ailerons (even when the plane is stationary!), while in a helicopter moving the stick to the left would always tilt the rotor disc. However in a tiltrotor neither is true - not to mention, we need to control the aircraft when it is transitioning between hover and cruising state!
So, just exactly how does one pilot a tiltrotor aircraft?

With a bit of practice.

This video explains it nicely starting at about 1:20 in.
Also see here.
This is a pretty interesting article on flying it as well. It really seems to depend on mode but its summarized here nicely,

Climbing into the front seats of the Osprey definitely does not
  produce the most graceful entrance: it requires some contorting around
  the armrest, center console and overhead panel. The cockpit is
  dominated by four night-vision-goggle-compatible, six-inch-square
  multi-function displays that allow access to flight, navigation and
  system information. Mechanical flight controls consist of a center
  control stick, thrust control lever (TCL) and rudder pedals. The
  control stick functions as cyclic control while in conversion and
  helicopter modes, but steadily fades into a traditional airplane
  control stick as the nacelles transition to airplane mode.
The TCL
  moves fore and aft just like an airplane throttle, unlike a helicopter
  collective. It does operate as a collective control, however, and
  becomes a traditional throttle during the transition. While this may
  seem counter-intuitive to helicopter pilots, it actually makes a lot
  of sense, because regardless of the mode of flight, youre always doing
  the same thing: controlling the thrust vector. Forward on the TCL in
  helicopter mode is the same as raising the collective in a helicopter,
  and vice versa. The first couple of hours for helicopter pilots
  transitioning to the Osprey highlight a bit of collective dyslexia,
  but very quickly it never becomes a further problem. A spring-loaded,
  knurled rotary knob on the TCL that lies in contact with the pilots
  left thumb controls the nacelles. Roll the thumbwheel aft and the
  nacelles rotate to the vertical, roll it forward and the nacelles
  continue to the downstops. The controls follow the hands on throttle
  and stick, or HOTAS, concept, and have all the controls necessary on
  them to control multiple systems on the airplane.


text:
Would the Gee Bee be allowed to fly today, with so little forward visibility?
When looking at the Gee Bee:

Source
I wonder what the pilot is actually able to see, when rolling on the runway, when flying level, or trying to locate a possible emergency landing place. It seems this aircraft puts the pilot and the people on the ground at risk.
Wouldn't a mirror improve safety, or is there a hole in the bottom of the fuselage?
How could such an aircraft be allowed to fly at the time? Did flying it only require to hold a recreational private pilot license or additional qualifications? Would it be allowed today in Europe, or in the US, based on existing regulation?

I'm not aware of any law that requires a specific amount of forward visibility, just that flight can be made with visual reference to the ground, and for this plane, a tailwheel endorsement.

The cockpit seems small in proportion to the fuselage, but I don't see why you would expect the visibility to be any worse than it is for many other cockpit-over-fuselage arrangements.

Take a look at the Vought F4U - it had an even longer nose.

The reasons such aircraft (including Spirit of St.Louis as pointed out by @ymb) were allowed to fly are explained in another answer. Quoting from it:

You don't need a panoramic view to land, ... Navigation is easy enough through side windows ... and you can just yaw the plane left and right for the times you do want to line up with something.
Even today, tail draggers have basically zero forward visibility on the ground so they taxi by watching the edge of the pavement with the occasional sharp s-turn to check ahead.

As for certification requirements, you can get an experimental certificate and fly it. FAA issues experimental certificates for racing aircraft:

§21.191   Experimental certificates.
Experimental certificates are issued for the following purposes:
(e) Air racing. Participating in air races, including (for such participants) practicing for such air races and flying to and from racing events.

GeeBee R2 was flying as recently as 2001 (not that recent, i know), so I'm pretty sure someone can fly it if they are upto it. FAA requires special pilot authorization for this kind of aircraft:

What type of experimental aircraft requires an FAA issued authorization?
This information applies to pilots of aircraft to which the FAA has issued Special Airworthiness Certificates for the purpose of Experimental under Title 14 CFR section 21.191 and are one of the following:

"Large" aircraft (more than 12,500 pounds),

Turbojet powered, or

Airplanes that have a VNE (never exceed speed) in excess of 250 KIAS and more than 800 HP.

What would be the requirements to meet today to obtain the airworthiness certificate for this aircraft, e.g. what level of visibility, and what would be the limitation for use? e.g. limited to exhibition or racing.

@mins The replica was tested according to FAA AC 20-27G Certification and Operation of
Amateur-Built Aircraft. It underwent 25 hours flight testing. You can go through that. As for visibility requirements, I'm not able to find anything specific. though I suspect that requirements for other aircraft will apply here too.

This YouTube video shows the view from an RC model of the plane.

In case the video is lost: It demonstrates very good visibility, well in excess of FAA regulations. Only a small sector of forward view is blocked.

While the nitpickers might say this is not an answer to the question, I think it is a great contribution to the issue. +1

While the nitpickers would argue the question is one of FAA regulation, the nitpicker's nitpickers would argue that the question is based on the assumption that forward visibility is almost zero, whereas the video demonstrates otherwise...  :-)

As an extra contribution, other videos showing the GeeBee taxiing demonstrate that the tail lifts relatively very quickly, enabling the pilot to have a more level view of the runway ahead.

Another nitpicker could argue that if the linked video is ever deleted or becomes unavailable, then this answer becomes meaningless. A description in text, or one or two well-chosen still images from the video uploaded to imgur, could easily rectify that.


text:
What should I do to prevent confusion for pilots due to the long distance between edge units on taxiways next to rapid exit taxiways?
We are at Hurghada Int’l Airport (HEGN) and we have a problem in three rapid exit taxiways named V5, V6 and V7 with taxiway V:

Moving to V5 for example, if you measure the distance between the first taxiway edge light unit on V before the entrance of V5 and the first edge unit on V after the exit of V5, it is 280 meters long. This may look like missing units to the pilots as you know the correct distance between edge taxiway lights is 60 meters.
So how can I solve that problem? For reference, we don’t have taxiway centerline lights, that’s why that problem occurred.

Could you add a bit more context to explain exactly what the problem is? Do you often get pilots making a wrong turn?

an issue with the longitudinal spacing of taxiway edge light units on main taxiway V at the intersection with rapid exit taxiways V5, V6, and V7. Since the standard spacing between taxiway edge light units is 60 meters, this presents a gap that exceeds the standard interval.
This issue has arisen because there are no centerline lights installed on the taxiway. The visibility level is not less than 550 meters, so centerline lights are not required. I don't recommend installing supplemental centerline lead-on lights on taxiway V in advance of the rapid exit taxiway intersections.

No any complains with pilots but it is an issue that should be fulfilled accordance to the aviation authority.

If no pilots are making mistakes here, I fail to see why this is an issue. If the aviation authorities require you to do something, surely they have explained to you exactly what needs to be done?

I would recommend you do some research using key words "problem statement".

Looking at the intersection V5, you are suggesting that taxiway edge lights are missing. The angle of the exit creates a large open taxi area where there is no edge to taxiway V on which to install lights. What you have is done correctly. If pilots or authorities complain about confusion your only option is centerline lights on taxiway V.

This is what you tell the aviation authority: The distance between any two edge taxiway lights is a multiple of 60 meters. The distance between any two adjacent edge taxiway lights is 60 meters, except where a light must be omitted due to interference with another ground feature, such as a crossing taxiway.
You could also say: Edge taxiway lights when present are spaced on a repeating interval of 60 meters. Some edge taxiway lights may be omitted from this sequence where they would interfere with other ground features, such as crossing taxiways etc.

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text:
What are the specific features of TY-90?
The wikipedia article about the TY-90 anti-helicopter missile mentions that China developed the TY-90 specifically as a helicopter-based anti-helicopter missile. This differs from the solution adopted elswhere, namely adapting MANPAD missile for use on helicopters.
While most articles are prolific on the feature that make the missile a anti-helicopter missile (all aspects, dual UV/IR, warhead...) I can't find information on what are the features that make it specifically suited for use in helicopters?
I'm troubled because my understanding is that any fire-and-forget missile could be used on any platform (if size and weight is ok). So why do the Chinese need a purpose-built missile?

The TY-90 is considerably larger than a MANPADS - 1.9m long and nearly 25 kg in weight.
MANPADS are restricted by human size and strength to about 15 kg for the missile.
A larger missile translates into more kinematic performance and a larger warhead, which allow more range and better kill probability.

Thanks for the answer, sorry that th equestion is closed


text:
Is it practical to descend in a lost communication scenario?

This is a follow-up to my previous question here -
  Lost radio communication after take-off in IMC - what are my options?.
  Please refer to that to understand the scenario of this question.

If, for e.g, at one point during the flight, I see a minimum en-route of 9000' and so I climb up to 9000' from 8000'. Now, if the next segment has a minimum en-route altitude of 3000', shall I go back to my previously held altitude of 8000' (which became my assigned altitude before and is now the maximum amongst all the other available altitudes) or shall I maintain 9000' and consider it to be my "new" assigned altitude?
Apart from a regulatory perspective, I'd also like if someone could weigh-in on practical perspective - would it be better in terms of traffic separation for ATC, etc.
So the question really is - is it practical to descend, or do the regulations want you to descend and hence ATC does not expect you to descend?
I've had different answers to this particular question from different Flight Instructors, and hence looking for a definite answer.

You may put you scenario here rather than refer to the previous question. The story seems to be: I take off from untowered aerodrome KSSI, fly IFR (KSSI SSI V441 MONIA GNV OCF V581 DADES KTPA). I was cleared by FSS: Climb and maintain 4000 expect 8000 20 mins after departure. I realize I lost my comms, I squawk 7600. And then explain. Note you need to fly the highest of assigned altitude, expected altitude, and MEA (FAR 91.185). So 8,000 (not 9,000) unless MEA forbids. Also note that ATC will try to talk to you on nearby VOR audio.

It is my understanding you would descend back down to 8000' since that is now the highest altitude.

The regulations are clear on the altitudes to fly so I'm not sure why you think that it might be "practical" to ignore them and do something else? Those regulations exist to make your actions predictable to ATC, so if you do something else then you're going to create some confusion. Unless of course you enter VMC, in which case you can land VFR if you decide it's the most practical option. If you want to give more details of the different opinions you've heard then maybe we can help to clarify them.

Maybe you should add in your question what country you are asking about. Rules can all be different in different countries...

The rules on this are well-established and clear. They are laid out in FAR §91.185(c)(2). In particular, you should fly at an altitude which is (emphasis mine):

At the highest of the following altitudes or flight levels for the route segment being flown:
(i) The altitude or flight level assigned in the last ATC clearance received;
(ii) The minimum altitude (converted, if appropriate, to minimum flight level as prescribed in §91.121(c)) for IFR operations; or
(iii) The altitude or flight level ATC has advised may be expected in a further clearance.

For each segment of your route, you evaluate the minimum, assigned, and expected altitudes, and fly the highest. In this case, you'd fly at 9,000 feet on the leg where the MEA is 9000, and then descend back to 8,000 when the MEA drops to that altitude or lower.
If you lose your radios and squawk 7600 (or stop squawking altogether), ATC will expect you to fly those altitudes. They will provide separation between you and all other IFR traffic at 9,000 feet on the first leg, and 8,000 on the second leg. The only exception to this is if you encounter VFR conditions, in which case you should maintain VFR, land, and call ATC so they stop holding the airspace open for you.
In this situation, you should not worry about inconveniencing ATC or other traffic. We have these rules specifically set up so that nobody trades paint if a radio fails. ATC will manage IFR traffic separation for you. "Better in terms of traffic separation" happens when everybody follows the same plan.


text:
How is an airplane towed/recovered after an emergency off-field landing?
I understand that there may be many different scenarios and situations, and that a similar question has some answers to this, but I was wondering if there are answers with existing examples. Some different scenarios are as follows:
General Aviation Airplane:

Landing on the beach
Landing on a road/interstate highway
Parachute Deployed Landing (Cirrus SR22) in a dense forest
Ditching in water

Large Jet Airplane:

Landing on the beach
Landing on an empty road/strip (such as the Gimli Glider)
Ditching in water (such as the Hudson landing)

I understand that the first priority is saving the souls on board. However, after that, the second-most important priority would be to recover the airplane as it is definitely a very valuable asset to the owner/company.
For all the above scenarios, what is a proven/tried method of recovering and towing while ensuring that the aircraft (or parts of it) can be put to use again?

Depending on the country, the aircraft cannot be recovered by the owner until the NTSB (US) or the investigating organization releases it. It is then typically "owned" by the insurance company, not the owner, at that point. How or if it is transferred back to the owner depends on the extent of the damage. In the case of water landings, many times the aircraft is not recovered (at least for GA) unless there is a compelling reason to do so.

The Gimli Glider was flown out of that airfield after some slight repairs were made to that plane.

Here is a timelapse video of the recovery of the "Hudson Miracle" 1549 A320, the aircraft itself is now in a museum.

@RonBeyer I understand the legal procedures. Let's assume that after such a landing on a road for GA airplane, all the paperwork etc. is taken care of by the insurance company and the Aviation Authority. What happens after that? I don't think it's something one can just tow on the road. Is it carried by a chopper? Is it torn down (if it is wrecked badly) and the parts towed individually?

@RaajTram Usually for GA aircraft, the wings are removed and it is trailered. If it is flyable, there have been rare instances of authorities allowing the aircraft to take off on a stretch of road or an open field. I can't think of any instance where an off-field landing of a large jet didn't involve cutting up the fuselage and hauling it off as a total loss... For some GA accidents, the wreckage wasn't recovered because it was too difficult or hazardous.

@SMSvonderTann That's interesting. I'm surprised how they managed to fix the nose gear and other external damage and take-off

After TACA flight 110 landed on the levee they changed the damaged engine at the site and it took off the adjacent road

This question seems very broad... it would be good to split at least GA and large jets into separate questions, as they will be very different. Maybe even further splitting the scenarios, depending on how much detail you are looking for.

@RonBeyer The "owned by the insurance company" part assumes the aircraft is totaled, right? Landing on a highway doesn't necessarily total an aircraft. I seem to recall an incident not far from where I live where a GA pilot ran out of fuel and landed on a highway. The aircraft was 'recovered' by getting some avgas in a tank from a local airport, refueling the aircraft on the highway, and taking off from the highway. And, like Tom said, there was a similar result even with a 737 in TACA 110.

@reirab Or at least severely damaged where the insurance company is contacted. If you read policies closely (even auto policies) the insurance company usually has the right to seize (for lack of a better word) the property when a claim is made against it. Obviously this requires a claim being made in the first place... What you are describing is the difference between an incident and an accident.

For a lot of GA planes, landing on a road, field, or other such surface is a matter of course.  Back in the '70s, my employer kept his Cessna in his barn, and took off/landed from the road in front of his house.  I've made any number of landings on e.g. dry lake beds.

This really breaks down into a few categories:
Flyable aircraft, Flyable location
For general aviation aircraft or airliners that are in flyable condition (or only require minor repairs) and are located somewhere you can fly them out of the usual solution is to offload as much weight as possible, take off, and fly to an airport.
There are countless cases of light GA aircraft taking off from roads or fields after precautionary landings.  In the large aircraft world the Gimli Glider flew out of the racetrack, and TACA 110 took off from a nearby road.

Flyable or Salvagable aircraft, Unflyable location
If the aircraft is flyable but can't take off from its current location, or if the aircraft is salvageable but requires more extensive repair work than can be accomplished where it is, alternative transport methods need to be considered.
For light aircraft the usual solution is to disassemble the aircraft, load it on trucks, and take it to an airport where it can be reassembled and repaired to be flown away.
This is also an option for most airliners, and was in fact the original recovery plan of action for TACA 110 that I mentioned above.
Disassembly, trucking, and reassembly is generally more expensive than flying the aircraft out of wherever you landed it, but aircraft insurance policies generally include provisions for this because simply leaving the aircraft where it is will often be even less economically attractive.

Wrecked aircraft, wherever it is
If an aircraft is largely intact but unlikely to fly again in the near future (or ever) due to damage the recovery operations are effectively the same as above, but they're dictated by legal and financial requirements: Environmental regulations may preclude leaving a wrecked aircraft wherever it landed, and financially the components and materials may be salvageable or valuable as scrap. As long as the costs of recovery are less than the profit to be made (or the fines to be incurred just leaving the aircraft where it is) and the hazards involved in the recovery are manageable the aircraft will usually be recovered.
On land the aircraft (or pieces of it) will once again be loaded onto trucks to be transported somewhere where they can be examined and then stripped/sold.
Aircraft in relatively shallow water will be generally recovered by dredging or hoisting with crane if doing so is feasible. This gets most of the "big stuff" (wings, fuselage sections, etc.) though in a badly damaged aircraft smaller pieces may be left where they are. (The recovery of US Airways 1549 is particularly spectacular in this regard, as it's hoisted basically intact.)
Aircraft in deep water are often not recovered - it's simply not feasible from a financial or risk management standpoint (sending people down to an extreme depth just to recover a machine that's going to be cut up and sold for scrap is hard to justify).

Regarding the last paragraph, the usual reason when aircraft are recovered from deep water is not selling them for scrap, but rather accident investigation (and, if there were people still inside, potentially return of the occupants' remains to their families.) For example, AF447, QZ8501, and the attempts for MH370.

@reirab I didn't consider recovery for accident investigation "in scope" for the question, but yes that's often a reason to spend the time and money dredging up an aircraft from deep water. (As a private operator if my Cherokee sank in a trench somewhere and I got out I'd say just leave the thing there, but if the NTSB wants to spend the money doing the recovery I'm game as long as I'm not footing the bill!)


text:
Raymers Installed engine equation
I am currently modelling an aero engine and require the weight breakdown of the individual components, I have the dry weight of the engine and need to verify the pylon and nacelle weight.
Raymer has presented an equation for the weight of the installed engine in his book
W installed engine = 2.575 *W engine^0.922*Nengine.
Does this include the pylon ?

That's an interesting question, +1, but you should provide some details (in which study or book), and also explain why you're unsure, in spite of having read the study.

It would help if you would post the equation that you're asking about.

I can see that you want an answer. But how significant would your error be either way?


text:
Would the FAA let me fly a plane with no windows?
Back in 2014 Airbus patented several designs for airplanes with no cockpit windows.
Would the FAA let someone (fully-certified for IFR) fly an airplane with no cockpit windows?

Are you certified for IVR?

Good point!  I've clarified my question.

Do you mean to replace the out the window view with cameras or to have no OTW view at all- just instruments including or not including synthetic vision?

Yes, drone pilots do that all the time.

If such an aircraft were to obtain a type certificate, and if you subsequently obtained a type rating in that aircraft, then the answer is yes.
Both aircraft and you would have had the proper testing and certification necessary to operate together and the FAA would let you fly that airplane.
Otherwise the answer is no.


text:
What are (wing area)/(weight) ratio in birds and airplanes?
I like to know the usual ratio of wing area over weight in birds and airplanes?
I mean if total weight of an airplane is 100 Lbs(Kg), how much should be the least total area of it's wings?
And also like to know this ratio in birds, if there are information about?
I need this ratio to estimate the wings size, if I want to create an aircraft to lift different weights(it's weight + load weight).
For example, if a person with 150 Lbs wants to fly, how much should be the least area of his wings?

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question.

Unmotorized? Look at a hang glider for example.  http://www.hanggliding.org/wiki/Choosing_your_first_wing shows wing area of 16.2m^2 for 60-100kg pilot. 70kg is abut 150 lb.  So 16.2/70 = 0.23 m^2/kg.  The conversion to get sq ft/lb is left as an exercise for the  reader.  Motorized, you can find similar small ultralight aircraft, wing areas are not a whole lot bigger, empty weight <= 155kg, small frame, limited fuel, and 1 cylinder engines. See the  "ATF light trike" here as an example  http://www.northwing.com/atf-trike.aspx

From 'The Simple Science of Flight', by Henk Tennekes, MIT Press
https://mitpress-request.mit.edu/sites/default/files/titles/content/9780262513135_sch_0001.pdf

Everything but a swallow, African, European, or otherwise, laden with a coconut or not....

Swallows are mentioned in pages 15 and 24 of the reference...

A Cessna 172S, which is a fairly recent airplane (less than 20 years old) with 4 seats, has a Wing Loading of 14.7 Lbs/Sq ft. In your terms, 0.068 sq ft/lb.
A 1977 Cessn 150, 2 seat trainer, has a wing loading of 10 Lbs/Sq ft. Or, 0.1 sq ft/lb.
A 1973 Cessna 177, a sleeker version of the 177 (more streamlined, no struts), has a wing loading of 14.4 Lbs/Sq ft. Or, 0.069 sq ft/lb
An adult male barn swallow has a wingspan of 32–34.5 cm (12.6–13.6 in) and weighs 16–22 g (0.56–0.78 oz). Lets say the extended wings are 3" front to back for simplicity, as I don't see any numbers on wing size.  Then 13.6 inch x 3 inch = 40.8 square inches, or 0.28 sq ft. Pick an average weight, 0.67 oz = 0.041 lbs, and the result is 0.28 sq ft/0.041 lbs = 6.68 sq ft/lb.
Similar math can be done for other birds.  Go pick one and work it out.

"A Cessna 172S, which is a fairly recent airplane" The -S may get a pass (1998), somebody but calling the 172 airframe recent makes me reach for my wooden stake and silver cross :)

Also, you forgot to specify whether this is an European or African swallow.

Well, Cessna is still making them, and folks & schools are still buying them.  https://cessna.txtav.com/en/piston/cessna-skyhawk#_model-top Not sure what the current model letter designation is.

Latest model? Not available new anymore "... a diesel-powered model, to be designated the 172 Skyhawk TD ("Turbo Diesel") starting in mid-2008. The planned engine was to be a Thielert Centurion 2.0, liquid-cooled, two-liter displacement, dual overhead cam, four-cylinder, in-line, turbo-diesel with full authority digital engine control with an output of 155 hp and burning Jet-A fuel. In July 2013 the 172TD model was canceled due to Thielert's bankruptcy. The aircraft was later refined into the Turbo Skyhawk JT-A, which was certified in June 2014 and discontinued in May 2018."  From Wikipedia

@CrossRoads The C172S is still in production.

Thanks, I just couldn't tell if it was S or something else from their website.

But there are a lot of other aircraft types out there.  I'd wager that a sailplane (with or without water ballast) has a much lower wing loading than a 172, while a 747 (152 lb/ft²), B1 (167 lb/ft²), B2 (67.3 lb/ft²).  Likewise, a hummingbird probably has a much different wing loading than a albatross.


text:
When does a passenger plane leave the cruising phase?
I have Flightradar24 data here. Which are in the following structure.
latitude    longitude   altitude    lastUpdate      velocity
....
49.34240    8.77556 19650   2018-11-01 09:38:09     415
49.35040    8.77490 19550   2018-11-01 09:38:15     414
49.35617    8.77442 19475   2018-11-01 09:38:20     414
49.37045    8.77308 19275   2018-11-01 09:38:24     412
49.38132    8.77208 19125   2018-11-01 09:38:30     411
49.38964    8.77132 19025   2018-11-01 09:38:34     410
49.39796    8.77054 18900   2018-11-01 09:38:39     409
....

This is data for a specific flight. My goal is to split the flight in two parts.

Pt 1: Airborne -> Exit cruise time / start Approach
Pt 2: Approach Start --> Touchdown

Well, I am not to deep into aviation. My naive approach would be:

If the flying altitude has dropped continuously over the last k
  points, then the first timestamp is the approach start.

Now I ask myself, in what period of time does a passenger plane leave the cruising phase?
This is different from the pilot calculating the top of descent explained here: What is a rule of thumb for when to begin a descent?
How much ft altitude does an airplane usually lose when it comes out of the cruising phase? And how long does he need for that?
The background is that I want to understand how long a passenger plane usually takes to land at a specific airport from the approach phase. Optimally, I then see that on days when the airspace over the airport is fuller on other days, times, the approach phase takes much longer. Or do you think there are other factors playing an important role? Or is the approach not so correct. Maybe it would be better if I do not look from Approachphase, but from the time where a certain radius was reached. 80km before arrival?

What you're trying to identify is the "top of descent" point. Clue: Planes try to descend in glide mode as much as possible to save fuel, so the location of the TOD and the rate of descent are linked to the glide ratio of the plane. See What is a rule of thumb for when to begin a descent? and all the linked questions (column at the right of the question) for an overview.

Note that aircraft sometimes climb/descend during the en route phase, typically due to weather or traffic.  It may be difficult (if not impossible) to reliably distinguish between this and the start of a typical stairstep descent on approach.

In a similar project I worked on years ago, we defined to use as top of descent the last point of the horizontal segment preceding the first descent segment that undercut 20000ft. Intermediate level-offs up to I believe four minutes duration were still considered part of descent segment. For flights never exceeding 20000ft we used the last horizontal point at highest altitude where at least five minutes level flight were attained (matching that airline’s short flight cruise planning policy), and if that condition wasn’t matched either, we simply used last point at highest altitude obtained.

What is the purpose of this exercise?

@ymb1 I don't see how this is a duplicate. That question is, basically, "When should I, the pilot, choose to start my descent?" This question is, "How can I, an observer detect that the descent phase has begun." The pilot of the flight may or may not be following the guidance given in answers to the other question.

@ymb1: "led me to conclude OP is after the TOD, but apparently not", of course the OP is after the TOD, and the other question explains how the pilot selects the TOD, but that's not the answer to the OP own problem, it just explains the context.

The background is that I want to understand how long a passenger plane usually takes to land at a specific airport from the approach phase. Optimally, I then see that on days when the airspace over the airport is fuller on other days, times, the approach phase takes much longer. Or do you think there are other factors playing an important role? Or is the approach not so correct. Maybe it would be better if I do not look from Approachphase, but from the time where a certain radius was reached. 80km before arrival?

@OrgosMos Radius should be a good one to measure airspace efficiency - after all, it will yield total ground speed through the terminal area around the airport. Bear in mind the last 20km or so are flown straight towards the runway in use (normally against prevailing wind) so this common point (normally the designated final approach fix for that approach procedure), not the airport, should be the centre of your measurement circle (for each individual aircraft if multiple runways are in use).

The distinction when you cross from one phase to another is somewhat arbitrary.  However, you can slice this multiple ways depending on your goal:  The approach phase could be considered to start at the first point of a published STAR, (Standard Arrival) the initial approach fix of a published instrument approach procedure, when the pilot descends below the class A airspace, i.e. <18,000' MSL, or the first ATC assigned vector off the filed route.  (Or some radius as you suggested...)
If you really want to dig into this my recommendation is to go to a local pilot shop and purchase a set of Approach Plates/STARS and a high altitude IFR chart for the area you are interested in.  You will likely need help interpreting them if you are unfamiliar with these publications.
If you then have a specific question on your airport of interest there are plenty of experienced people here who are willing to help, but a full education on high altitude IFR enroute and terminal arrival procedures is beyond the scope of this forum.
In lieu of that, if you don't want to take that kind of time to learn something foreign but have a specific airport you are interested in analyzing, please let me know.  I have a full set of United States procedures loaded in my iPad EFB and would be happy to post a screenshot with an interpretive description of where I might consider the approach phase to begin for a specific airfield/runway combination.
There are many many different arrival procedures nation wide, into airports of varying levels of traffic congestion.  Without a better understanding of your actual goal the question as stated is almost to broad to answer, especially for someone who is admittedly "not too deep into aviation".  Good luck with your project.

+1 for the observation that such lines are "somewhat arbitrary".  Going farther & calling them "entirely arbitrary" might even be more accurate.  Some STAR's (Denver comes to mind) begin so far from the field that you're often at cruise altitude for the first several points on the STAR.  Maybe you step down 2000' for ride or traffic, then cruise level for 5 minutes, then step down another 2000', then cruise level, then get a "pilot discretion" descent clearance, then get cleared "descend via", and then later you actually start down. When did "the descent" start? Call it what you will!


text:
Changing the interception angle in autopilot for OBS or VOR in the G1000 from 30 to something more aggressive
I am flying the G1000 and have noticed that if I program in an OBS TO or a VOR TO (this might apply to FROM as well), the angle at which I fly relative to my programmed in bearing is 30 degrees.
For example, if I am flying 180 and want to fly to an airport nearby with an OBS setting of 250 and put this in and turn on NAV, the plane will turn to 220, which indeed will intercept 250 at some point. But if I am not far from the airport I am wanting to approach at 250, this interception will get me too close to the airport, by the time I reach 250. So I would like a more aggressive interception angle than 30... maybe 90? How would I do that?

The manual does not state any way to do that. You can adjust the vertical navigation flight path angle, but not the horizontal intercept angle. You'll have to fly that yourself. In addition, in this case you should be telling the G1000 where you want it to go, rather than which heading to go. If you just tell it the heading it will never figure out what it has to do to get you to that nearby airport, so in this case it is working as advertised.

Ok. I have scoured the online documentation and asked my instructor this and concur I don't have a clear answer to this either.
By "adjust the vertical navigation flight path angle", are you referring to the fact this gets indirectly set by my setting the VSI or IAS I want to fly to reach a new altitude?
I don't quite follow what you mean by telling the G1000 where I want to go rather than heading. I thought the OBS does exactly the former, where I have a waypoint and I am saying "Approach this point on this radial". Problem here being I reach it when I am upon it.

Search the docs for flight path. It will tell you. OBS just tells the G1000 in which direction you want the aircraft to go, not to what waypoint. I would strongly suggest you at least take one of the many online courses for the G1000.

Actually I did take a flight in the sim with an instructor. We did not spend a lot of time on the OBS. Perhaps I misunderstood it. So the point of it is not to actually reach that waypoint but rather use that waypoint almost like it is a VOR station and choose the radial along which to fly, which comes down then to a direction dictated by the TO radial coming out of that waypoint?


text:
What are the regulations involved in aerial advertising?
I often see small aircraft flying at a low altitude around beaches and the like with banner advertisements trailing behind them on a rope sort of structure. I have a few questions about them:

What sort of permissions does it require (FAA) ?
Is this hazardous in any way (I believe there would be some
restrictions on this type of flight but is there an inherent danger
involved in such flights) ?
Are there any limits as to what altitude they are allowed to reach and
any restricted air space that they are only allowed to fly in ?

The short answer is that you need to get an FAA waiver (special permission), as required by 14 CFR 91.311:

§91.311   Towing: Other than under §91.309.
No pilot of a civil
aircraft may tow anything with that aircraft (other than under
§91.309) except in accordance with the terms of a certificate of
waiver issued by the Administrator.

(91.309 is about towing gliders, so banner towing comes under 91.311.)
You need at least a private pilot certificate to tow banners - sport and recreational pilots aren't allowed to do it - but since most banner towing is paid work, in practice the pilot will usually have a commercial certificate (this is from the waiver instructions):

When banner tow operations are conducted for compensation or hire, the
pilot must have at least a limited commercial pilot certificate
(without an instrument rating) and at least a valid second class
medical certificate. An instrument rating is not a requirement for
this operation.

The FAA has a complete guide on banner towing and the waiver issue process has more administrative details. There don't seem to be many specific airspace requirements apart from the obvious, e.g. no towing in a TFR zone. But the FAA can impose more or less any conditions that they like as part of a waiver, and it will include the approved times and areas for operation.
(As an aside, it's widely believed in the pilot community that the permanent stadium and Disney TFRs are really there to prevent banner towing, not to increase security.)


text:
Why do aircraft models end their life as freighters?
There seems to be a tendency that aircraft models are more long-lived for cargo than for passenger service.
For example, both the A300 and MD-11 are all but extinct in scheduled passenger service, but FedEx and UPS still fly scores of them.
My understanding is that the reason an aircraft model goes out of service is not that the planes are too old to possibly continue flying, but that it becomes economically favorable to replace them with newer models with better fuel or maintenance economy etc. Shouldn't those considerations apply with equal strength to cargo planes?
Which factors contribute to passenger aircraft obsolescence that do not apply equally well to freight?

Self-loading cargo just prefer new, spangley planes :)

Too speculative for an answer IMO, but: With the possible exception of the lowest of the budget airlines, passenger aircraft need a cabin refit every now and then.  This isn't cheap, and for an old plane without much value and that may be coming up to retirement for other reasons (efficiency) it might not be worth it.  Especially when you consider the cost of taking it out of service for some time. A freight refit must be much cheaper.

Passenger operations are overwhelmingly scheduled whereas cargo is on demand and thus less easy to plan. The upside: A cargo plane crew will fly quite diverse routes. The downside: Capacity utilisation is worse. This shifts the balance of cost such that fixed cost must be lower than what is possible with scheduled services. As a consequence, older aircraft with lower depreciation cost are preferred. With them it is less costly if they don't move for some hours or even days.
Caveat: Integrators (freight companies which ship door-to-door like FedEx or UPS) run their freighter fleet like a scheduled service, but still their utilisation is lower. Please see @TomMcW's answer for a detailed analysis.
Operating cost are more important when the aircraft flies as much as technically possible. Especially low cost airlines need to operate hardware with the lowest operating cost possible so they survive the next price war. Premium airlines also have a need for the newest planes: This makes them look premium and more safe. Cargo operators have no such incentive to prefer the newest airplanes; they profit from the need of passenger airlines to sell their older hardware off.
Next caveat: Older airplanes are at least as reliable as new ones when maintained well. The need of premium airlines for newer planes is only to impress their ignorant customers, not to increase real reliability numbers.
The Economist wrote in April 2013 about Norwegian Air Shuttle:

As fuel prices soared in the mid-2000s, and competition from the likes
  of Ryanair and easyJet intensified, Mr Kjos realised that the only
  route to survival was to buy lots of new, fuel-efficient planes to
  achieve economies of scale, and to base some of them in cheaper
  countries. Boeing and Airbus are straining to keep up with booming
  demand for new aircraft, so by reserving a large chunk of their
  delivery slots NAS is denying potential rivals the chance to refresh
  their fleets with better planes.

If an airline is large enough, ordering new aircraft has the added benefit of saturating supply and denying the same aircraft to its competitors. If oil prices are high, this airline will have won the price war before it started in earnest.

Very interesting. Many factors I would have never thought of.

I will say this -- most cargo (esp. large/irregular stuff) is on-demand/chartered.  (Scheduled cargo flying is the domain of parcel carriers such as UPS and FDX.)

Cargo is not necessarily an on demand service. Except for a few ad-hoc flights, FedEx flights are regularly scheduled daily flights. Most of them either 4 or 5 days a week depending on destination

Wouldn't the airline be saturating demand rather than supply? The only one who can saturate a supply is/are the supplier(s). The airline could be considered to be consuming the supply, too.

As TomMcW stated UPS, FedEx, Atlas, and others, are Part 121 scheduled carriers.

@DoktorJ I think it's correct as is. The consumers have control over demand, so can increase it or decrease it at will (with limitations, of course). They can't saturate demand because they can just ask for more. Conversely, the consumers do not have control over supply, but they can saturate supply by making demand equal to or greater than the fixed (from the consumers' point of view) supply. And suppliers can not saturate supply because they control the supply and thus can just increase supply. But suppliers can saturate demand by supplying as much or more than is demanded.

Does this mean that airlines typically amortize their aircraft as much as possible over their run in passenger service? How else would they be able to sell them off into a secondary (freight) market at prices that would work as described?

@AnthonyX: Airlines like Ryan operate like a car rental firm: They buy bulk with huge rebates and sell the aircraft on a few years later when they can sell them (now individually) for almost the same amount they paid initially. Premium airlines hold on to their aircraft for longer and hand them down to second-tier airlines after much of their value has been written off.

@TomMcW: Does the FedEx schedule ask for one return flight per day and destination or do they fly throughout the day? FedEx flies all parcels into Memphis once and then flies them out a few hours later. As far as I understand their business model, the rest of the day the aircraft sit idle.

They don't usually just sit at the dest. I don't pretend to know exactly what they're doing with them, but they move around during the day. At least the ones from our ramp do. It's not always the same but our MEM plane often goes back to Memphis for day sort and comes back to MCI in the afternoon. Our IND plane sometimes goes to TUL then to the hub at AFW then back to MCI.

@PeterKämpf if you look a FedEx at Memphis they operate like any airline hub. Big arrival push, unload, sort (like pax transfers) and reload and a big departure push. They are scheduled and you could depend on that to ride their jumpseats to get to work.

@casey: Yes, and they need this to get the highest possible number of flights in and out in a short time. But I wonder whether their fleet utilisation is anywhere near that of passenger airlines.

@PeterKämpf , FedEx need to ensure that their flights connect with their trucks on the ground that such that they can start delivering the packages, JIT arrival.
Another reason why cargo airlines fly with older equipment is that they do not fly to the fancy passenger airports but to remote cargo hubs were there are less noise regulations. Here in Europe they also fly mostly at night since the drivers can't deliver packages then.

@Brilsmurfffje: Why can't European drivers deliver packages at night?

Freight planes fly less.  Since they're not in constant use like a passenger airliner, it makes more sense to pay less up front (fixed costs).

And flying the planes less means they have more buffer..in case an airplane is not available another can be slotted in. Another plus point is older airplanes typically need more attention. Less flying means more time to fix stuff.

If just like to add to the answers here and address a few inaccuracies made about air cargo. What you ask is true, some of the oldest aircraft out there are flying cargo, and most of the answers here are accurate, but some things are based on false assumptions.
The question mentions full size airliners so I am focusing on that. I use FedEx as an example because I'm most familiar with their system. Some things that apply to a large cargo-only carrier may not apply to other parts of the industry.
To clear up a misconception:
Efficiency and reliability are just as important in freight
Fuel burn and cost to operate are just as important Freight operators are no less concerned with the cost of flying and upkeep of their aircraft than the passenger companies. When fuel costs go up it affects them the same as everybody. When fuel prices soar the carriers often implement "fuel surcharges" on top of their regular pricing to help make up the cost. They want the most efficient aircraft they can fly. It's harder to make money if your planes are gas-guzzlers.
Keeping planes in service is just as important Flight delays are just as costly for freight as passengers. Some have pointed out that the inconvenience to passengers has a higher cost to the passenger industry. Not from the freight operators' point of view. They don't want planes broken down any more than the passenger airlines. Being stranded as a passenger is much more of an inconvenience than not receiving your Amazon order. But when you delay a passenger flight you have hundreds of unhappy customers. When a freight flight is delayed there could be thousands of unhappy customers. Some of those customers are very large shippers. If you screw up too many times they may jump ship for another carrier. Losing a big source of revenue like that can have quite a sting financially.
There's more than just airside. For the passenger airlines that move freight and other airport to airport services mechanical delays impact their schedules. But for freight companies that also deliver the freight there is an entire network of employees, much larger than the airside component, that can be left unproductive. I work as a driver and when one of our flights comes in late for mechanical reasons (or any reason) there are 50 of us and 10 or so handlers and sorters standing around on the clock twiddling our thumbs. Our ramp serves 3 large and 4 small stations, so that one late flight can mean overtime for a couple hundred people.
Reasons why older aircraft remain in freight service longer:
Longer life span due to different patterns of use
Cargo aircraft spend more time on the ground. On average passenger airlines have an easier time keeping their aircraft busy. The difference may not be as much as some suspect, though, at least in the case of FedEx. It's hard to find solid numbers on this. Taking a look at flight histories on flightaware of a random sample of FedEx planes and comparing them to similar passenger airliners I found that the passenger aircraft made about 6-8 flights per day whereas the FedEx planes were making 4-6 daily flights. An aircraft on the ground is still not making money. This is no less true for a freighter than a passenger plane.
Fewer short hops The advantage of air freight is that it can cover long distances in a short time. For shorter distances freight can be moved by truck in a similar time for a lot less money. Airlines often make several short hops to keep the seats full. Inter-continental transport is where the air advantage is truly significant. Air is often the only way to move freight in a timely manner. The wear and tear on a plane depends much less on the time or distance it flies. The biggest factor for airframe longevity is pressurization and depressurization cycles. The most wear and tear on engines, brakes, undercarriage, etc come at takeoff and landing. So the longer the flights the longer a plane will last.
Passenger comfort
Passengers prefer newer planes Whether it be the perception of more safety or the greater comfort of newer, quieter aircraft or the bells and whistles often built into newer planes, passengers would rather fly on newer aircraft. Freight doesn't care so much.
Interior wear and tear One place where aircraft have a lot of wear is the interior. This is mostly cosmetic, but seat cushions sag, upholstery wears out, windows get scratched and yellowed. These things can be repaired, of course. But there comes a point when it's time to retire it, but the airframe still has many years left in it. Freighter conversion is often the best option.
Growth and availability
Regulation Up to the 1970's the air freight industry was almost an afterthought. Airlines were heavily regulated by the Civil Aeronautics Board. The board controlled routes and pricing. They also regulated who could fly airliners. Many believed the CAB was overly focused on passenger service and air freight in the US floundered. Even in this situation, the airlines began buying dedicated freighters. The 747 was designed around the concept of being a freighter. The cockpit was put on top to make it easier to load freight containers resulting in the famous "hump."
Deregulation When FedEx was formed in the early 1970's as a freight only airline they started out with a fleet of small Dassault Falcon 20 jets. By the late 70's they had outgrown the Falcons, but were still not able to move to bigger aircraft due to regulations. In 1977 the air freight industry was taken from CAB control and deregulated. There was suddenly a need for larger freighters. The handful of companies manufacturing airliners can only make so many planes a year, so they began buying passenger 727's and DC-10's as they became available and converted them to freighters. Fortunately for them deregulation hit Branniff Airlines hard and they soon folded making their large fleet of 727's available for freight conversion. FedEx flew these 727's long after the passenger services retired them. Several of the DC-10's are still flying today.
Rapid growth FedEx fleet of dedicated freighters is the 5th largest in the world, including passenger lines. The industry expanded rapidly and the passenger airlines had a head start on aircraft purchasing. The freight services joined the new aircraft purchase game. FedEx was the launch customer for the newly minted MD-11, getting the first six aircraft off the line. They have orders still pending for new 777's and were one of the first to order the A-380, although the economic downturn of 2008 caused them to cancel that order and change it to 777's. But aircraft are built slowly and there are a lot of airlines needing them. To keep up with the needs for large freighters they can't afford to let go of the DC-10's in the fleet. The need for efficiency still affects freight operators so they had their DC-10's upgraded to MD-10's which only require two pilots. As they are able to replace them with newer, more efficient aircraft they will eventually be able to send the DC-10's to their retirement homes in New Mexico.

Unable to find any good data on number of flights and flight hours, I got creative. This is really unscientific, but I looked up flight histories month to date on 6 random similar aircraft (B777) from United Airlines and FedEx. Here is a table of the results:

I was a bit surprised that the number of flights were so close. The UA aircraft spent slightly more hours in the air and the FedEx aircraft spent slightly more time on the ground
Like I said, such a small sample from two airlines is not very scientific. I'm not sure if this would bear itself out over the entire fleet, but I can't find numbers anywhere.

Very interesting points. Thanks for taking the time to write it.

Can you conclude that freight aircraft spend more time on the ground? You say that FedEx's planes do fewer flights per day than passenger planes, but you also say that they do fewer shorter hops. (+1, btw.)

@DavidRicherby Working on that. Will get back to you

@DavidRicherby Out of curiosity I got creative and made a little table from a random sample. Took a while, so I appended it onto the answer

@DavidRicherby I might note that volume at FedEx is down this month. Typical for February.

Aircraft models are longer lived in their cargo versions because that is where they end up. Most of the cargo aircraft are older passenger aircraft that have been converted into cargo aircraft. As a result, the capital costs are lower compared to that of the passenger aircraft. Your aircraft is cheaper, and as a result, you have more leeway in direct operating costs (compared to passenger aircraft, that is).
As a World bank report notes:

The average operating cost of an aircraft in flight is computed by dividing the direct operating costs plus  capital  costs  by  the  number  of  hours  of  aircraft  operation.
The capital costs for air freighters are a relatively small part of total operating costs because of their age. The common practice of all-cargo airlines and combined carriers is to purchase used passenger aircraft and  convert them by adding loading ramps. After 10 years of operation, the price of an aircraft will have decreased by at least 50 percent and after 15 years by 65-70   percent ...

In the end, it all boils down to cost efficiency- whether the savings due to improved fuel efficiency of newer models offset the higher capital costs required to acquire them in the first place; as long as this expenditure is not justified, the aircraft will continue to be old and this trend is expected to continue in the near future:

1800 passenger aircraft are forecast to be converted for cargo use over the next 20 years , 60% of which will be narrowbody types, predominantly
from A320, 737 and 757 families.

Another thing about cargo aircraft is that the pricing is dependent on the LOS (Level of Service) offered- higher cost for a better LOS. By offering improved LOS, the cargo airlines can improve their direct revenue unlike the scheduled passenger flights.
Also, it is not entirely true that all cargo aircraft are old. Integrators (Companies which manage an operate the entire logistical chain from door to door) prefer newer aircraft as they are more efficient, while pure cargo airliners prefer the other way. For example, the Boeing 767 frieghter line is solely dedicated to FedEx.

This doesn't explain any difference between cargo and passenger operations, as far as I can see. If it is economically favorable for cargo operators to fly older models simply because they are cheap, why don't passenger operators take advantage of this too?

@HenningMakholm They do, a lot of budget airlines purchase older aircraft to keep costs low, or standardize on a single type of aircraft (like Southwest).

@RonBeyer: The clever budget airlines use the newest planes with the lowest operating cost, so they have an advantage in the unavoidable price war. The saying goes: If you live by price you die by price.  Look at Vueling and Ryan Air for proof.

As far as most cargo planes being former passenger planes that have been converted. FedEx was the launch customer for the MD-11. The first six off the line were freighters. The majority of the A-300 fleet came with that "new plane smell." The Cessna Caravan was created for cargo service.

@TomMcW Giving some examples of brand-new cargo planes does nothing to contradict a claim about most cargo planes. Are you claiming that most cargo planes were actually built for cargo?

@DavidRicherby I spent some time last night trying to put a more specific number to that but it was crazy tedious. Was having to go through plane by plane. Specifically for the FedEx fleet it is probable that there are more aircraft purchased new than used and getting more so all the time. When I get some time I'm going to write an answer with some further details. There's no doubt, though, that FedEx is flying some of the oldest planes around. There are still DC-10's in the fleet

@TomMcW OK, but FedEx is still just one carrier. It's a very large carrier but there are hundreds and hundreds of small carriers with just one or two planes.

@DavidRicherby FedEx has the 5th largest fleet in the world, including passenger airlines. The company has a number of subsidiaries, but those are all ground freight. They have to keep the express system separate from the others for complex regulatory reasons.

As you want to offer your passengers maximum comfort and a quiet airplane which also should be economic and efficient passenger aircraft are replaced by newer models quite often whereas for cargo airplanes the comfort doesn't matter at all. "Fracht motzt nicht, Fracht kotzt nicht" (Cargo doesn't shout, Cargo doesn't spill) as we say here in Germany illustrates this. As long as it's rentable for a cargo airline to use an airplane it'll be used.
After thinking about this it sounds quiet logically to use the replaced passenger aircraft as cargo planes if they are big enough, doesn't it?

That doesn't explain why, say, Ryanair (which as far as I hear doesn't care one whit about passenger comfort or satisfaction -- on the contrary, they cater to passengers who think the worse they are treated the better a deal must they be getting) doesn't fly a fleet of old cheap planes from of the same kind cargo operators use.

@HenningMakholm For Ryan Air to be profitable they need to be efficient. That means efficient aircraft are needed.

@HenningMakholm As well as aircraft efficiency, also remember that Ryanair are in direct competition with Easyjet, FlyBe and the like.  On the Kano model, a minimum level of comfort is a "must-be" factor, whereas cost is a "one-dimensional" factor.  So you won't gain customers on a budget airline by being more comfortable, but if your planes give more than a certain level of discomfort then you will actively lose customers, even though they might cost less.

I would say that the factor that differs most between passenger and freight is the "Value of Reliability".  A passenger flight has a very high cost of unscheduled unavailability.  Especially in hub and spoke networks, a plane being late or cancelled causes follow-on problems as passengers miss connecting flights. There is a cost in customer satisfaction, and extra expenses.  And, presumably, customers value their own time or their employers do, so time wasted is value lost.  In freight, reliability is also important, but the costs of being delayed may not be nearly as high.  When it is, newer planes will be used.  Factors that make freight reliability worth paying for are:  perishable cargo, cost of being out of stock, need for cargo for a production process, and high value cargo which has an intrinsic interest cost.  A need for a parcel to make it to a hub on time would also make the Value of Reliability higher.
Transportation analysis uses the Base Cost, Value of Time and Value of Reliability to help determine what trade-offs are more tolerable.  The terms "Willingness-to-Pay" and "Willingness to Accept" are used, and market participants are surveyed in a sophisticated way to figure out what values they put on trade-offs.
Older aircraft have more time needed for scheduled and unscheduled maintenance, and can't be tightly scheduled if there is a high cost for not arriving on time.  But, they are cheaper.  As far as "Value of Time", new and old planes are about the same speed in the air, but the time between flights can be longer for freight -- one per day, not several.  Maybe the freight company can schedule your flight when they have a plane in position.  Maybe this doesn't work when you're shipping racehorses--expect to pay more.
Some kinds of freight fit the characteristics that the freight's time isn't really valuable and being late won't cause too much of a problem.  For these products, customers expect a corresponding lower base cost than the cost of a passenger ticket per pound.  Most freight also has a lower Value of Time than passengers -- your package is OK with a 3-day journey, but passengers would not be.  To meet the the lower cost necessary to satisfy freight customers, who have a lower penalty (loss of economic utility) for delays, older planes can do the job.
Older planes are not "unreliable" in that safety is not directly related to aircraft age, it's just that some components are going to have to be removed more often than on a new plane, and therefore "availability reliability" will be lower.

The special cargo versions of various aircraft models are even produced longer as the passenger versions.
Passengers demand a certain level of luxury, which can't be easily included in older models. Mostly it needs a new wiring plan and some other changes. This makes it cheaper to create a new model and incorporate new technology and other demands of the airlines as well.
Cargo isn't very demanding and the interior of cargo planes are much simpler. Older aicraft models are cheaper as the new ones. New aircraft models come with a cargo version after 5 to 10 years.

Usually a passenger aircraft needs a lot of attention and budget to keep it satisfactory for passengers. Besides cleaning and decoration the engine sound also needs to stay smooth.
There is a difference between old engine and new engine sounds. Old engines, may be still running perfectly, yet it is bound to make extra noises.
An older aircraft engine and interior will make some extra sound. (You know if you ever traveled some old crafts from some Asian countries. They keep using the aircraft as passengers careers even after the expiry period is over.)
While these noise and other issues are no problem if the aircraft is carrying cargo instead of passengers.

You can't really say that engine noise is not an issue with freight. It's not the cargo that complains; it's the neighbors. That's probably a big reason FedEx retired their 727 fleet. Those things were so loud they set off all the car alarms in the parking lot. They spent a lot of money developing hush kits for them. But in the end they were probably finding more and more places they couldn't fly due to noise ordinances.


text:
Would better location tracking for airliners improve safety?
Once in a while an airliner just up and disappears -- typically over ocean -- and lengthy and expensive search efforts have to be launched in order to find it. The most famous cases in recent years were of course AF447 and MH370.
It is hard for the bemused public to understand how something as large and technologically advanced as a wide-body jet can just go missing. Some people end up on websites like this one, asking why we can't just do such-and-such to make sure planes can always be found. The answers typically center around how such-and-such would actually be very expensive, and probably not 100% effective anyway. This doesn't really seem to convince most questioners, who wonder how important a bit of cash ought to be when there are lives at stake.
The purpose of this question is to clarify whether lives are actually at stake.
Suppose we had a magical technology which at negligible cost could give us reliable access to the exact flight path of an airliner in trouble anywhere on Earth. How much safer would that make flying?
For definiteness, let's assume that the magical technology gives us position samples at 1-minute intervals at a one-hour delay, so it won't be useful for real-time purposes such as ATC.

just 2 gone missing over the thousands that also did the crossing? Also "GPS tracking" doesn't work like it does in the movies; a quarter sized device will not let a laptop on the other side of the world track it to any kind of precision.

@ratchet: The question is: Even if tracking technology did work like it does in the movies, would it make us any safer?

@ratchet, actually, the SPOT beacon, battery powered for a satellite uplink, about the size of a deck of cards, does exactly that.   I always fly with mine.  Why the big airliners don't use it is beyond me.

@abelenky some of them do - or at least are capable of the functional equivalent: planes with the Iridium datalink service could easily send their GPS position/speed/track information periodically. As  for why they don't do it as a matter of company policy or regulation - probably cost (The cost-benefit tradeoff doesn't work out. We've had one airliner vanish mysteriously in how many years.

@abelenky: More specifically, MH370 did fly with satellite uplink that would send it's position (at least occasionally; this thing does not report every second like ADS-B) if it wasn't disabled, most likely by deliberate action by somebody on board. If it had any better reporting, it would most likely have suffered the same fate and thus be just as useless in the end.

Would better location tracking for airliners improve safety?
Very simply, no.... us knowing where AF447 was would have made exactly zero difference to the fact that it crashed into the ocean at 400+ knots.
Would we have had a better idea of where to find it? Most probably. Would it have saved any lives? Absolutely not.
As always, these things come down to cost-benefit, and the simple fact is that GPS tracking would prevent very, very few accidents. The only possible benefit would be that it may help us find the black box or wreckage, assisting in the investigation which may just possibly reduce the chances of a similar accident occuring in future. Note that of the two examples you gave, though, only MH370's black box hasn't been found (yet)... so even this benefit is tentative.

Recovering MH370's cockpit voice recorder probably won't help - it will have overwritten the interesting part of the flight. The flight data recorder would probably be of some interest if it wasn't somehow disabled early on. Even so, prospects for a major improvement in safety seem slim.

True: there's no guarantee that having accurate location information would actually mean the black boxes would be found, nor that they would contain useful information: nor that they wouldn't be found anyway.

"Would it have saved any lives? Absolutely not." It could certainly save lives. Not people onboard the accident aircraft, mind you, but people on future flights, which could be made safer based on the experiences learned from the accident.

@J.Hougaard I don't see how a bunch of GPS data from MH370 could prevent an exact repeat of the same incident

@JonStory I'm not saying it could, but the aviation industry does have a great record of improving safety by learning from previous incidents. Generally speaking, more data is likely to improve safety

In case of a crash/incident where an airliner ends up on the ground, it would allow for faster search & rescue. I.e. lives may be saved since rescue arrives asap.
Second situation would be in case of an airliner cutting all communication and being used as a weapon (think 9/11). Then tracking would allow ATC/military to keep track of the airliner. But with an 1 hour delay it would probably not be relevant as a preventative measure.

If it's in danger of hitting anything, it must be over land, and if it's over land we can track it with radar.

There is not full radar coverage over earth´s entire landamass. And radar cannot identify a specific plane, only give information about the position of a chunk of metal in the air.

"In case of a crash/incident where an airliner ends up on the ground, it would allow for faster search & rescue.": Faster than finding the existing ELT which is already mandatory over water? "radar cannot identify a specific plane": Wrong, this is exactly what SSR does.


text:
What are the rules in the UK about overflying built-up areas for fun?
I live in North London.  It's not like we don't have a fair amount of noise pollution from the sky, with helicopters, and jets landing at LHR with a stupidly shallow glide path angle.
But in addition, quite often on a Sunday afternoon I hear this buzzing of a small propeller plane.  It often seems to circle the area quite a few times before eventually going away.  We had one today: I looked at it, and (this time) it was a small plane with two props, one on each wing.  I have no idea what type of aircraft it might have been.  It was flying low enough for the buzzing to be distinctly intrusive, and therefore affecting potentially tens of thousands of people.
It may not have been a flight simply for fun. I presume there's no way of finding out... but does anyone know what the rules are for just randomly deciding to fly over a built-up area like this for the fun of it?
PS I have friends in Cambridge and you often see small light planes overflying that town. They have a friend who owns a light aircraft, and once or twice my friends have said "oh, yes, that's Jim's plane", and have confirmed he does it for recreation.

Thanks... that appears to be a site for tracking large jets landing at LHR. I'm aware of such information but my questions not about large jets.

@mins OK... thanks... well I had a look again. I haven't quite worked out how to "go back in time" with that map thing.  Secondly, this aircraft I saw earlier circled around locally quite a few times, and I've no particular reason to think that its destination or origin was LHR, or even any big airport.  But I'll give it a few more minutes trying to work out whether it's there...

Can you give a specific time when you heard this plane? And as to jets landing at LHR with a stupidly shallow glide path angle None of the glide path angles at LHR are less than the standard 3°. In fact some of them require an angle as high as 5.24°. The only one I’ve ever heard of that’s higher is 5.5° at LCY.

No, I don't have a specific time, and the question is about the CAA rules, such as they may be.  As to LHR glide path angles, I don't know where you're getting your information from, but this is COMPLETELY INCORRECT.  But this particular question is not about the almost unimaginably cynical politics of BAA's and CAA's management of London airspace and their utter nonsense about noise pollution: that could be another question...!

If it's a twin then it's very possibly a traffic spotting flight out of Stapleford, they overfly traffic hot-spots and report them to radio news stations.

@Mike Rodent - The Heathrow glide slopes, ILS and PAPI angles are described in the documents at http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task=blogcategory&id=94&Itemid=143.html  .  This is what ATC and visiting pilots will use and they are all set at the usual 3°

@mins - your SERA comment is the correct answer, you should write it up

Does anyone know what the rules are for just randomly deciding to fly
  over a built-up area like this for the fun of it?

In UK, the master document for air regulations is The Rules of the Air Regulations (ROTAR). UK being still part of the EU, they refer to a common set of rules at the EU level, the Standardised European Rules of the Air aka SERA (qui sera sera...)
UK ROTAR state a minimum flyover height, one for VFR, one for IFR:

The minimum heights for VFR flights shall be those specified in
  SERA.5005(f) and minimum levels for IFR flights shall be those
  specified in SERA.5015(b)

and SERA says:

SERA.5005:
(f) Except  when  necessary  for  take-off  or  landing,  or  except
  by  permission  from  the  competent  authority,  a  VFR  flight shall
  not  be  flown:

(1) over  the  congested  areas  of  cities,  towns  or  settlements
  or  over  an  open-air  assembly  of  persons  at  a height  less
  than  300  m  (1  000  ft)  above  the  highest obstacle  within  a
  radius  of  600  m  from  the  aircraft;
(2) elsewhere  than  as  specified  in  (1),  at  a  height  less
  than 150  m  (500  ft)  above  the  ground  or  water,  or  150  m
  (500  ft)  above  the  highest  obstacle  within  a  radius of 150 m
  (500  ft)  from  the  aircraft.

So for VFR: 150 m, but 300 m in populated areas.

SERA.5015:

(2) elsewhere  than  as  specified  in  (1),  at  a  level  which
  is  at  least  300  m  (1  000  ft)  above  the  highest  obstacle
  located within  8  km  of  the  estimated  position  of  the  aircraft.

And for IFR: 300 m above obstacles.
As you see this is not very high. In addition, as you mentioned, aircraft landing or taking off will indeed fly below these minimums.

Jets landing at LHR with a stupidly shallow glide path angle

Landing cannot be done at high angle, the proper slope is the result of two elements:

Aircraft cannot afford a high descent speed, since the vertical rate determines how hard the landing gears will touch the runway and whether they will be damaged.
Aircraft cannot slow down under a given speed without losing their capability to create lift. This minimum speed and the previous maximum vertical speed determine a maximum slope.

This glide slope has been chosen so that it is compatible with all sorts of aircraft approach speeds and vertical rates.
A standardized value of 3° is common. But when the approach area is limited, or when this is desirable to maintain a higher altitude a longer time to reduce noise on the ground, then this angle can be increased to 3.5 or 4.5°, or even 5.5°, this is the case for some approaches at Heathrow as @TomMcW told us.

Source
Alternatively, some airports have lower angles, down to 2.5°.

Thanks, that's very interesting.  I'm absolutely amazed that, seemingly, anyone can just choose to fly over an area like North London any time, and indeed I'm surprised there aren't more recreational flights if that's the case... are you sure there's nothing else to prevent this activity?  Surely they can't do it at night, etc.?

Re glide paths to LHR.  This is the bane of my life and I am not alone. I have learnt more than I need to about the murky politics of this and the point about me is that I live 15 miles ("as the plane lands") from touchdown at LHR.  The sine of 3 deg is 0.0523 which therefore means planes should be 0.79 miles above my house minimum.  I claim that this is NOT THE CASE.  BAA and CAA have for decades indulged in this fiction according to which noise from jets landing at LHR only "significantly" affects a couple of miles around the airport. NONSENSE.

More on the politics: BAA/CAA cannot ever admit the reality about noise pollution from jets landing at LHR, because that would then mean there would have to be serious efforts and regulations to "share the misery" evenly over the whole of the "catchment" area for LHR.  For this reason planes are still concentrated into "corridors" in arbitrary fashion, and moreover those corridors could be changed at any time.  Only a Third World country would tolerate the amount of noise pollution which Londoners routinely take for granted.

@mikerodent: Yes this can be done at night, unless there is a curfew. But I believe curfews are for airport landing/takeoff, not for just flying around. This can even be done under "VFR at night" rules (no specific navigation instrument required). If you ask me, yes this is abusive.

Yes, and it's not just the noise... VFR, in such a congested airspace as London, with innumerable helicopters, etc. As I say this appearance of a light plane is very infrequent currently, but if this ever changed I would be seriously concerned about the possible consequences of a crash in such a densely populated area.  I'm pretty astonished.

"anyone can just choose to fly over an area like North London any time" - it depends which part of North London.  Here's an airspace chart for that area - http://nats.aero/blog/wp-content/uploads/2014/07/Southern-England_Ed40_v2_Standard.jpg - In the London and London City CTAs you'd need a zone clearance from Air Traffic.  Around Lippets Hill you'd expect a minimum safe altitude of 1500 feet and a ceiling of 2500 without a clearance into the TMA.  You may be seeing light aircraft flying in and out of Stapleford and Elstree....

... also, there is this: "SERA.3105  Minimum  heights
Except when necessary for take-off or landing,  or except by permission from the competent authority, aircraft shall not be flown over the congested areas of cities, towns or settlements or over an open-air assembly of persons, unless at such a height as will permit, IN THE EVENT OF AN EMERGENCY ARISING, A LANDING  TO BE MADE WITHOUT UNDUE HAZARD TO PERSONS OR PROPERTY ON THE SURFACE. The minimum heights for VFR flights shall be those specified in SERA.5005(f) and minimum levels for IFR flights shall be those specified in SERA.5051(b)"

@Mike Rodent - there are arguments against your claim that the glide slopes being used are not those that are published but I think it would have to take place in chat rather than here, and probably wouldn't change anyone's mind.  BTW, I spent some years living in Richmond so I'm familiar with the noise levels on final :)

@DaveGremlin: How is SERA.3105 applicable to UK?

@mins  The old CAA ANO Rule 5 (the low flying rules) had a requirement that an aircraft should be able to 'glide clear' of built up areas etc.  SERA 3105 replaces this with the 'landing without undue hazard' paragraph.  SERA came in as part of the change from the JAA to the EASA and is replacing (slowly and with some exclusions) the previous UK Rules of the Air.  Chapter and verse is in this document: https://www.caa.co.uk/uploadedFiles/CAA/Content/Standard_Content/Commercial_industry/Airspace/Rules_of_the_air/Files/20171012SERAABC_ANO_PermissionsExemptions_ROTA2015Consolidation.pdf

There are two types of rules I'm aware of that apply here:

Low flying prohibitions. These include the 1,000 feet rule and the land clear rule (a.k.a. the glide clear rule). In a built up area you must be 1,000ft above the highest obstacle and able to glide clear of the built up area in the event of a power-unit (engine) failure. If you have more than one power unit (like the multi-engine you mentioned), then you can continue flying with a single engine failure and I suspect that removes the need for glide clear.
Controlled airspace. North London is covered by controlled airspace from the surface up, so even if complying with the rule above you still need permission from air traffic control to fly there. Here's a diagram of the airspace over London, which includes TMZ, CTR, CTA and TMA (all different types of controlled airspace, all requiring permission):

Unless you in an emergency, flying into controlled airspace without permission (clearance) will get you into a world of trouble, anywhere from a stern phone call or retraining to getting your license rescinded depending on location and circumstances.
While most private pilots avoid London, there's nothing to stop you asking for a "zone transit" and if they aren't too busy they might even say yes. Here's a video of a private pilot getting permission to fly right over North London at 2,000ft. Incidentally it's a twin engine like the one you mentioned. He's on his way somewhere and clearly experienced, so more likely to get a clearance.
Without clearance you could possibly still fly over some of North London - down to Finchley or thereabouts. Without a multi-engine aircraft you could argue you're able to glide to Totteridge Common. Here's the edge of the airspace for reference: